The German colony of German East Africa was bordered by Portuguese Mozambique to the south and the British colonies of Rhodesia and British East Africa to the southwest and north. Across its western border of Lake Tanganyika lay Belgian Congo.

Britain dispatched 4,000 Indian troops which reached the colony on September 1, 1914, followed by 8,000 in October.

General Paul von Lettow-Vorbeck commanded a colonial army of German and African (Askari) troops in a guerrilla campaign. To assure control of Lake Tanganyika he armed a steamer. In January 1915 he attacked at Jassin and decided to instead fight a defensive campaign to tie down British troops, dividing his forces into Northern, Western, and Southwestern groups. By the end of 1915 he had 3,000 Europeans and 11,300 African troops.

Attacks by German forces destroyed 32 trains and 9 bridges between May 1915 and March 1916. Under South African General Jan Smuts, the British staged the Morogoro Offensive between March and September 1916 clearing northern East Africa of German forces. An Anglo-Belgian advance from the northwest captured Tabora at the western end of the rail line to Dar es Salaam and the coast.

Despite the Allied advance, Lettow-Vorbeck successfully withdrew to the south leaving behind destroyed rail lines and bridges. He resupplied from a coastal blockade runner.

Disease forced the evacuation of many British troops in late 1916, replaced by African and West Indian troops. The primary British force grew to 35,000 men.

In the face of a British offensive in September 1917, Lettow-Vorbeck withdrew into Mozambique with 2,200 troops. Pursued by the British, they trekked 1,250 miles (2,000 km) before returning to East Africa on September 28, 1918.

Learning of the European Armistice on November 13, Lettow-Vorbeck surrendered on November 25.